HOW DO I FIX A OIL LEAK THAT I CANT FIND WHERE ITS COMING FROM.

1991 Nissan 300ZX

When the oil leak began there was a lot of black smoke coming from the exhaust The car when started after I clean the spark plugs runs fine for bout half and hour then it starts to sound like it has a miss in the engine then the black smoke comes from the exhaust. What can be the problem with the engine. I bought a new engine seal kit for it and was wondering if changing the seals would fix the problem.

Easy enough to see a leak off a manifold, try placing cardboard under the engine area so the drippings will probably show. The black light system does work on small crack leaks like on a transmission case or a cracked engine block, good luck.